1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to systems for sub-aperture data imaging of double sided interferometric specimens, such as semiconductor wafers.
2. Description of the Related Art
The progress of the semiconductor industry over the last years has resulted in a sharp increase in the diameters of semiconductor wafers as base material for chip production for economic and process technical reasons. Wafers having diameters of 200 and 300 millimeters are currently processed as a matter of course.
At present manufacturers and processors of wafers in the 200 and 300 mm range do not have a wide range of measuring devices available which enable inspection of particular geometric features, namely flatness, curvature, and thickness variation, with sufficient resolution and precision.
As scanning of specimens has improved to the sub-aperture range, the time required to perform full specimen inspection for a dual-sided specimen has also increased. Various inspection approaches have been employed, such as performing an inspection of one side of the specimen, inverting the specimen, and then inspecting the other side thereof. Such a system requires mechanically handling the specimen, which is undesirable. Further, the act of inspecting the specimen has generally required binding the specimen, which can cause deformation at the edges of the specimen, increase defects at the edge, or cause bending of the specimen during inspection.
One method for inspecting both sides of a dual sided specimen is disclosed in PCT Application PCT/EP/03881 to Dieter Mueller and currently assigned to the KLA-Tencor Corporation, the assignee of the current application. The system disclosed therein uses a phase shifting interferometric design which facilitates the simultaneous topography measurement of both sides of a specimen, such as a semiconductor wafer, as well as the thickness variation of the wafer. A simplified drawing of the Mueller grazing incidence interferometer design is illustrated in FIG. 1A. The system of FIG. 1A uses a collimated laser light source 101 along with a lensing arrangement 102 to cause grazing of light energy off the surface of both sides of the specimen 103 simultaneously. A second lensing arrangement 104 then provides focusing of the resultant light energy and a detector 105 provides for detection of the light energy.
The design of FIG. 1A is highly useful in performing topographical measurements for both sides of a dual-sided specimen in a single measurement cycle, but suffers from particular drawbacks. First, the system requires minimum specimen movement during measurement, which can be difficult due to vibration in the surrounding area and vibration of the specimen itself. Further, the inspection can be time consuming and requires highly precise light energy application and lensing, which is expensive. The specimen must be free standing and free of edge forces, and the incidence geometry during inspection must be unimpeded. Access must be preserved under all incidence angles. These factors provide mechanical challenges for successfully supporting the specimen; excessive application of force at a minimum number of points may deform the specimen, while numerous contact points impede access and require exact position to avoid specimen deformation or bending during inspection. Further, edge support of the specimen has a tendency to cause the specimen to act like a membrane and induce vibration due to slight acoustic or seismic disturbances. This membrane tendency combined with the other problems noted above have generally been addressed by including most components of the system within an enclosure that minimizes ambient vibrations, which adds significant cost to the system and does not fully solve all vibration problems.
The cost of lenses sized to accommodate inspection of a full wafer in the arrangement shown in FIG. 1A are highly expensive, and generally have the same diameter as the diameter of the specimen, generally 200 or 300 millimeters depending on the application. Full aperture decollimating optics, including precision lenses, gratings, and beamsplitters used in a configuration for performing full inspection of a 300 millimeter specimen are extremely expensive, generally costing orders of magnitude more than optical components half the diameter of the wafer.
It is therefore an object of the current invention to provide a system for performing a single measurement cycle inspection of a dual-sided specimen having dimensions up to and greater than 300 millimeters.
It is a further object of the present invention to provide a system for inspection of dual-sided specimens without requiring an excessive number of binding points and simultaneously allowing free access for inspection of both sides of the specimen.
It is a further object of the current invention to provide for the single measurement cycle inspection of a dual-sided specimen while minimizing the tendency for the specimen to behave as a membrane and minimize any acoustic and/or seismic vibrations associated with the inspection apparatus and process.
It is still a further object of the present invention to accomplish all of the aforesaid objectives at a relatively low cost, particularly in connection with the collimating and decollimating optics and any enclosures required to minimize acoustic and seismic vibrations.
The present invention is a system for inspecting a wafer, including inspecting both sides of a dual sided wafer or specimen. The wafer is mounted using a fixed three point mounting arrangement which holds the wafer at a relatively fixed position while simultaneously minimizing bending and stress. Light energy is transmitted through a lensing arrangement employing lenses having diameter smaller than the specimen, such as half the size of the specimen, arranged to cause light energy to strike the surface of the wafer and subsequently pass through second collimating lens where detection and observation is performed.
The system further includes at least one damping bar, where the number of damping bars depends on the wafer repositioning arrangement. The effect of the damping bar is to perform viscous film damping, or VFD, of the non-measured surface of the specimen to minimize the effects of vibration in accordance with VFD, or the Bernoulli principle. Each damping bar is positioned to be within close proximity of the surface to be damped. The proximity between any damping bar and the surface of the wafer is preferably less than 0.5 millimeters, and spacing of 0.25 and 0.33 may be successfully employed. Smaller gaps provide problems when warped specimens are inspected. One embodiment of the current invention employs a damping bar to cover slightly less than half of the specimen when in scanning position.
Mounting for the wafer uses a three point kinematic mount. The mounting points include clips having spherical or semi-spherical tangentially mounted contacts, mounted to a support plate and arranged to be substantially coplanar, where the clips are adjustable to provide for slight irregularities in the shape of the wafer. The adjustability of the contact points provide the ability to hold the wafer without a stiff or hard connection, which could cause bending or deformation, as well as without a loose or insecure connection, which could cause inaccurate measurements.
Light energy is conducted through a beam waveguide and then strikes a deviation mirror, is redirected onto a parabolic collimation mirror by two further deviation mirrors. The deviation mirrors are oriented at an angle of 90xc2x0 relative to each other. The parallel light beam P reflected from the parabolic mirror reaches a beam splitter through the two deviation mirrors.
The beam splitter is formed as a first diffraction grating and is arranged in the apparatus in a vertical direction. The parallel light beam P strikes the diffraction grating in a perpendicular direction. A beam collector in the form of a second diffraction grating is disposed from the first diffraction grating and parallel thereto. Behind the beam collector two decollimation lenses are arranged at equal level. The light beams leaving the decollimation lenses are each deflected and focused onto two CCD cameras through various deviation mirror pairs and to an optical imaging system.
The beam splitter is supported transversely to the optical axis and includes a piezoelectric actuating element for shifting the phase of the parallel light beam P by displacing the diffraction grating.
A wafer or specimen to be measured is held on a holding device such that both plane surfaces are arranged in vertical direction parallel to the light beam P. The wafer is supported substantially at its vertical edge so that both surfaces are not substantially contacted by the support post and are freely accessible to the interferometric measurement.
A receiving device may be provided. Further, a reference apparatus may be provided which comprises a reference body having at least one plane surface. The reference body can be introduced into the light path between the first diffraction grating and the second diffraction grating in place of the semiconductor wafer or specimen to be measured by means of a traveller with a linear guide. The reference body is held so that its plane surface is arranged in vertical direction parallel to the undiffracted light beam P.
Modifications of the imaging apparatus and method are possible. A body having two precisely plane parallel surfaces may be used for the reference body, whereby both surfaces are measured simultaneously. However, the embodiment having a single plane surface of the reference body is more suitable.
In one arrangement, a light source initially emits light energy and strikes two mirror surfaces, which each direct light energy through a first collimating lens and simultaneously strike the two surfaces of the specimen. Light energy is thereupon directed through a second pair of collimating lens and to a second pair of mirrors, toward a focusing element arrangement, and a detector. A translation surface or mounting surface holding the contact points and wafer or specimen is fastened to a translation stage, which provides translation or sliding of the specimen within and into the lensing/imaging arrangement. The system first performs an inspection of one portion of the specimen, and the translation stage and wafer are repositioned or translated such as by driving the translating stage so that another portion of the wafer is within the imaging path. The other portion of the wafer is then imaged, and both two sided images of the wafer are xe2x80x9cstitchedxe2x80x9d together.
Other means for presenting the remaining portion of wafer or specimen may be employed, such as rotating the wafer mechanically or manually, or keeping the wafer fixed and moving the optics and imaging components. Alternately, scanning may be performed using multiple two-sided inspections of the module, such as three, four, or five or more scans of approximate thirds, quarters, or fifths, and so forth of the specimen. While multiple scans require additional time and thus suffer from increased throughput, such an implementation could provide for use of smaller optics, thereby saving overall system costs.
In a two phase scan of a dual sided specimen, at least 50 per cent of the surface must be scanned in each phase of the scan. It is actually preferred to scan more than 50 per cent, such as 55 per cent, in each scan to provide for a comparison between scans and the ability to xe2x80x9cstitchxe2x80x9d the two scans together.
Scanning and stitching involves determining the piston and tilt of the specimen during each scan, adjusting each scan for the piston and tilt of said scan, and possibly performing an additional stitching procedure. Additional stitching procedures include, but are not limited to, curve fitting the points between the overlapping portions of the two scans using a curve fitting process, replacing overlapping pixels with the average of both data sets, or weighting the averaging in the overlapping region to remove edge transitions by using a trapezoidal function, half cosine function, or other similar mathematical function. Background references are preferably subtracted to improve the stitching result. If significant matching between the scans is unnecessary, such as in the case of investigating for relatively large defects, simply correcting for tilt and piston may provide an acceptable result. However, in most circumstances, some type of curve fitting or scan matching is preferred, if not entirely necessary.
These and other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.